Energy question of the week: How much electrical power can be harnessed from tides?

Under the influence of the Moon's gravity, the water of Earth's oceans rises and falls twice a day. Powerful forces are at work all over the globe between every low and high tide – forces that can be harnessed to generate electrical power. Just how mature is the technology and what is the potential that tidal power stations could unleash?

This year, the latest large tidal power station, the Sihwa Lake Project on the West coast of South Korea, went into operation. Numerous other pilot plants are being set up around the globe in an attempt to meet the challenge of utilising the tidal currents in our oceans and seas. The new tidal power station in Korea will oust the world's largest such power station, situated near the town of St. Malo on the northern coast of Brittany, France, from its first place. Since 1966, the 24 tube turbines at the 'La Rance' tidal power station, each capable of generating ten megawatts of power, have been delivering a reliable supply of electricity. The unique layout of the Rance estuary causes the sea level to fluctuate by up to 14 metres between high and low tide. While the Korean power station is only able to generate electricity during a rising tide, the French turbines rotate during ebb as well as flood tides. This is made possible by the specially-shaped Kaplan turbine wheels that are capable of rotating in either direction of tidal flow.

Aerial view of the French tidal power plant in the Rance estuary near St Malo. Credit: dpa.

60 million tons of sea water every year

Interestingly, the massive power plant in the bay opposite the South Korean city of Ansan came into being through the pressing need to make a virtue out of a necessity. It started with a dam extending for almost 13 kilometres, separating Lake Sihwa, which has a surface area of 57 square kilometres, from the Yellow Sea. Originally, this dam was intended to drain new areas of land for agriculture, and to improve the regional water supply, but it turned out that this was not possible. The quality of water declined rapidly due to the influx of waste and sewage. Now, the South Korean government is converting the Sihwa dam into a tidal power station. During every flood tide, gigantic volumes of water flow into Lake Sihwa through the ten tube turbines, then flow back out during each ebb tide. This should not only revitalise the artificial lake, but it should also generate enough electricity for half-a-million households.

An elaborate control system ensures the circulation of up to 60 million tons of seawater a year, enabling the 254-megawatt power station to generate about 540 gigawatt-hours of electricity. To accomplish this, the Sihwa Basin needs to be drained through the sluice gates as quickly as possible during every ebb tide. As large volumes of flood-tide seawater build up in front of the dam, the water level rises by up to eight metres and this level of 'drop' enables the turbines and generators to be driven as water flows through them and into Lake Sihwa. The angle of the turbine blades can be adjusted continuously in order to generate the maximum amount of electricity over the longest possible period. With the Sihwa power station in place, South Korea will be able to more than triple the proportion of 'sustainable' electricity, that is power generated from renewable resources, from 1.4 to about 5 percent at one go.

World's largest tidal range

The Bay of Fundy, on the Atlantic coast of Canada, exhibits the world’s largest natural tidal range – a range of 21 metres has been measured in the past, making it an ideal location for a tidal power station. A small power station has been located at the site since 1984, in Annapolis Royal, although this is only equipped with a single 20-megawatt turbine. Since tidal power stations can be operated effectively with tidal ranges of five metres or more, there are also some small plants in China and Russia, although these only have a capacity of between one and four megawatts. In all, more than one million gigawatt-hours of electricity are generated from tidal power around the globe each year, spread over at least 100 locations.

The DLR Energy question of the week in 'The future of energy' Year of Science

The Federal Ministry of Education and Research (BMBF) has given the Year of Science 2010 the motto 'The future of energy'. For this reason the science journalist Jan Oliver Löfken will this year answer a question on the subject of energy in his blog each week. Do you have a question about how our energy supply might look in the future? Or do you want to know, for example, how a wave power plant works and how it can efficiently generate electricity? Then send us your question by email. Science journalist Jan Oliver Löfken will investigate the answers and publish them each week in this blog.

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Energy journalist Jan Oliver Löfken writes among other things, for the Technologie Review, Wissenschaft aktuell, Tagesspiegel, Berliner Zeitung and P.M. Magazin on issues involving energy research and industry. For DLR, he answered the Energy question of the week during the Year of Energy 2010.
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